The HCN/H\(^{13}\) CN total line flux ratio is \(\approx 19\)  and 42 for the \(J\) =1–0 and \(J\) =3–2 transitions, respectively. Assuming similar excitation conditions for both isotopologues and an elemental abundance ratio \(^{12}\) C/\(^{13}\) C equal to 66 (see below), close to the local ISM value [1]}, we then derive line opacities \(\tau _{10}\simeq 3\)  and \(\tau _{32}\simeq 0.8\) 
for the \(J\) =1–0 and \(J\) =3–2, respectively, i.e. the gas is moderately optically thick. The HNC/HN\(^{13}\) C takes lower values, \(\approx 11\)  and 16 for the \(J\) =1–0 and \(J\) =3–2 transitions, respectively, which implies opacities of about 5 and 4 for the main isotopologue, respectively, still under the assumption of a standard elemental abundance ratio.
